The present inventions are related to systems and methods for operating a servo system, and more particularly to system and methods for performing burst demodulation in a servo system.
A read channel integrated circuit (IC) is one of the core electronic components in a magnetic recording system such as a hard disk drive. A read channel converts and encodes data to enable magnetic read heads to write data to the disk drive and then read back the data accurately. The disks in a drive typically have many tracks on them. Each track typically consists of user data sectors, as well as control or “servo” data sectors embedded between the user sectors. The servo sectors help to position the magnetic recording head on a track so that the information stored in the read sectors is retrieved properly.
FIG. 1a depicts a data format of a servo data sector 100. As shown, servo data sector 100 may include a preamble pattern 102 which allows the system to recover the timing and gain of the written servo data. Preamble pattern 102 is typically followed by a servo address mark (SAM) 104 which is the same for all servo sectors. SAM 104 is then followed by encoded servo GRAY data 106, and GRAY data 106 is followed by one or more burst demodulation fields 108. GRAY data 106 may represent the track number/cylinder information and provides coarse positioning information for a read head traversing a magnetic storage medium. Burst demodulation field 108 provides fine positioning information for the read head traversing a magnetic storage medium. Burst demodulation field 108 typically includes sine waves written to a medium that can be used for retrieving head position information relative to the medium. Traditional systems use full rate demodulation where the frequency of the sine waves match that of preamble pattern 102. Thus, any timing acquisition done based on preamble pattern 102 may be applied to burst demodulation field 108. FIG. 1b shows the aforementioned servo data sector 100 incorporated as part of each of a number of tracks 160 each extending in a radial pattern around a radial magnetic storage medium 150. In an ideal case, a read head traverses an individual track over alternating servo data sectors and user data sectors.
When synchronizing to magnetic storage medium 150, data obtained using a read head traversing the medium is typically equalized to a desired target partial response by an equalizer configured as a continuous time filter (CTF) followed by a discrete-time finite impulse response (FIR) filter. In a synchronous system, the sampling of the CTF output signal uses timing information generated by a digital phase-locked loop (DPLL) locked to the symbol rate. The output samples of the equalizer are quantized to digital sample values (‘Y’ values) using an A/D converter (ADC). The ‘Y’ values are applied to a data detector (e.g., threshold detector or Viterbi detector). A SAM detector then searches for the SAM bit pattern in the detected data. Once SAM is detected, the GRAY code decoder decodes the data following the SAM data as GRAY data. The burst demodulation is timed with respect to the detected SAM data based on known lengths of the SAM and GRAY data. The detected SAM data thus serves as a reference for timing of the burst demodulation operation.
FIG. 2 depicts a prior art burst demodulation system 200 that may be used in relation to a servo system. An input signal 202 is received via an analog coupling stage 205, an automatic gain control circuit 210, and a continuous time filter 222. Input signal 202 intermittently includes servo data that is used to direct the sampling rate and sampling phase used for data within a given sector. A digital phase lock loop circuit 235 provides a clock output 237 that controls the points at which input signal 202 is sampled by an analog to digital converter 230. The phase and frequency of clock output 237 is adjusted based on an error signal provided by a phase/frequency detector 280. Phase frequency detector 280 generates the error and a slope based on the output from analog to digital converter 230. The error and slope signals from phase/frequency detector circuit 280 cause an adjustment to the phase and/or frequency of clock output 237, and continues to cause an adjustment until the error signal goes to zero.
In addition, the digital samples from analog to digital converter 230 are provided to a digital FIR filter 240 and to one or more digital interpolators 245. Digital interpolators 245 are operable to identify an incoming preamble signal and to determine the optimal phase/frequency for sampling the preamble. In particular, the processing of the preamble develops periodic boundaries (T) corresponding to the symbol rate defining the sampling times that are used in processing a subsequent SAM pattern and GRAY code pattern using a SAM detect and GRAY code detect circuit 255 to, among other things, identify the SAM incorporated in input signal 202. SAM detect and GRAY code detect circuit 255 provides a SAM found output signal 257 indicating that the SAM has been identified. A burst demod circuit 260 then seeks to identify the burst demodulation information incorporated in input signal 202.
In typical existing servo systems, processing a full rate burst demodulation pattern hinges only on proper assertion of the SAM found output signal. In particular, the burst demodulation information is found by counting a defined number of periodic boundaries (T) from SAM found output signal 257. FIG. 3 depicts such a situation where sampling to identify a SAM pattern 301 is indicated by vertical lines 303, 305 separated by four periodic boundaries (i.e., 4T). A SAM found output 307 is asserted coincident with sample 305. An intervening GRAY code 309 is decoded, followed by detection of a full rate burst demodulation pattern 311. The peak of a full rate burst demodulation pattern is an integer multiple (n) of periodic boundaries (T) (i.e., nT 313) from SAM found output 307. Of note, the sine of the sample is much greater than the cosine where the sample is taken near its peak. Such a situation results in a high signal to noise ratio.
This process of performing burst demodulation processing works very well where for full rate burst demodulation patterns as the peaks of the patterns occur an integer multiple of periodic boundaries from the SAM found signal. There are trends in the art, however, to use half rate demodulation patterns in place of the aforementioned full rate patterns. Such patterns offer a variety of advantages, but they do not typically align on periodic boundaries measurable from the SAM found signal. In some cases, while such half rate demodulation patterns offer some advantages, they can be difficult to detect and process.
Hence, for at least the aforementioned reasons, there exists a need in the art for advanced systems and methods for performing burst demodulation.